This invention relates to a process for the production of hard vinyl chloride resin foams and more particularly, it is concerned with a process for the production of cellular or foamed resin products having dense foams and uniform surface property with a high foaming efficiency or high expansion ratio.
Up to the present time, vinyl chloride resin foams have been prepared by various methods, for example (1) using volatile organic solvents, (2) using hydrocarbon compounds and (3) using foaming agents of decomposition type. However, these methods of producing foams by using foaming agents have some unfavourable disadvantages. In the method (1) using volatile organic solvents, for example, a disadvantage is unavoidable that the working efficiency is lowered due to the inflammability of a volatile organic solvent and the necessity of removing and recovering the residual solvent in foams and it is difficult to store the resulting foams for a long time because of the volatile organic solvent used. The method (2) using hydrocarbon compounds has also a disadvantage that a special means or apparatus for introducing a hydrocarbon compound under pressure uniformly into a resin is required, the selection of a suitable hydrocarbon compound depends greatly upon a resin used and a long time storage property is inferior. Furthermore, the method (3) using foaming agents of decomposition type has also a disadvantage that foam controlling is difficult because the decomposition rate of a foaming agent, in particular, of inorganic decomposition type is stepwise slow. Lately foaming agents of organic decomposition type have been widely used, since they are relatively stable in inflammability and storage property and easy to handle due to the form of solid powder, and furthermore, they can readily be mixed by a relatively simple mixing method and applied to various resins. The foaming agents of organic decomposition type have these many advantages, but, on the contrary, have some unfavourable disadvantages. That is to say, the quantity of the foaming agent must be within some limit, since if too little, no sufficient expansion ratio is obtained, whilst if too much, discoloration takes place due to the residual foaming agent and cells become large resulting in coarsening of the surface, in spite of that the expansion ratio is not increased so much. In order to overcome these disadvantages, it has been proposed to apply a radiation or bridging agent to a foaming agent-containing resin composition, thereby effecting the bridge making thereof (Japanese patent publication Nos. 19384/1973 and 4861/1973). According to this method, the expansion ratio is increased, but, on the contrary, several disadvantages are unavoidable that the bridged resin composition loses the intrinsic nature of thermoplastic resin, so that the workability is lowered and the molding or forming method is restricted, and sometimes encounters decomposition and coloration.
In the simplest method for the production of foams, in general, a resin and foaming agent are mixed and subjected to foaming in a suitable forming machine but, in this case, the cellular film strength must be sufficient enough to resist the gas pressure at the time when the foaming agent is decomposed. Therefore, the temperature at which a foaming agent is decomposed, gas quantity and gas pressure at the time when a foaming agent is decomposed, and melt viscosity of resin at the time when forming is carried out must be taken into consideration.
In the case of vinyl chloride resins, the elongation and viscosity of the molten resin are greatly changed in a forming machine, so satisfactory foams can hardly be obtained due to the uneven foamed structure and to the degassing phenomenon from the foam surface produced by breakage of the cell films. As a method to overcome these disadvantages there have been proposed various methods for modifying vinyl chloride resins by the use of plasticizers of modifiers miscible with vinyl chloride resins.
However, the inventors have found that these plasticizers or vinyl chloride resin modifiers are effective for holding the cellular film but should be used in considerably large amounts so as to obtain a satisfactory cellular film strength. That is to say, it is found necessary to use a plasticizer in an amount of at least 10% by weight, preferably 20% by weight based on the weight of a vinyl chloride resin. The foams obtained by the method using plasticizers in such large amounts show a markedly lowered melting point, whereby the object of the present invention to obtain hard foams cannot be achieved. When using a vinyl chloride resin modifier, on the other hand, it is also found necessary to use in an amount of at least 10% by weight, preferably, at least 15% by weight based on the weight of a vinyl chloride resin. The use of a modifier which is expensive in such a large amount is disadvantageous from an economical point of view.